Visibility tokens on types in `extern { ... }` blocks are no longer given to procedural macros

[alexcrichton]

I had wasm-bindgen/wasm-bindgen#2009 opened up on a project recently but this looks to be a regression I think. This regressed between nightly-2020-02-19 and nightly-2020-02-20, and bisecting further by commit reveals #69271 as the culprit. I'm not sure which of the PRs in that rollup though are the culprit here.

This can be reproduced by running cargo check from the crates/js-sys folder in the wasm-bindgen repository. Unfortunately this one probably isn't super easy to reproduce, so I'm hoping that someone more knowledgeable can hopefully take a look at this and have a better idea about what's going on.

There's definitely macros going on in #[wasm_bindgen] (the Memory type is synthesized by a macro), which leads me to think that #69211 may be the culprit? (cc @petrochenkov)

[petrochenkov]

The large parser PR from @Centril (#69194) could also cause something like this.

[petrochenkov]

I'll be able to look at this during the weekend, but it would be great if someone could come up with a minimal reproduction before that.

[alexcrichton]

Ok, wanted to get a gut-check to see if it was known what was happening. Did some reduction and it looks like the macro no longer gets the pub token for the input type.

Given this input:

pub mod wasm {
    #[the_macro::the_macro]
    extern "C" {
        pub type Memory;
    }
}

fn main() {
    let _: wasm::Memory;
}

then with this macro:

extern crate proc_macro;

use proc_macro::*;

#[proc_macro_attribute]
pub fn the_macro(_attr: TokenStream, body: TokenStream) -> TokenStream {
    let g = match body.clone().into_iter().nth(2) {
        Some(TokenTree::Group(g)) => g,
        _ => panic!("unexpected input"),
    };
    let tokens = g.stream().into_iter().collect::<Vec<_>>();
    assert_eq!(tokens[0].to_string(), "pub");
    return body;
}

it fails with:

$ cargo +nightly build
   Compiling wut v0.1.0 (/home/alex/code/wut)
error: custom attribute panicked
 --> src/main.rs:2:5
  |
2 |     #[the_macro::the_macro]
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: message: assertion failed: `(left == right)`
            left: `"type"`,
           right: `"pub"`

error: aborting due to previous error

error: could not compile `wut`.

To learn more, run the command again with --verbose.

On stable it yields the expected error[E0658]: extern types are experimental error which indicates that the pub token was fed to the macro.

If the #[wasm_bindgen] macro doesn't see the pub token it doesn't make the final result public, which is why the privacy error happened in the first place.

[alexcrichton]

(updated the issue title to reflect what appears to be the actual bug)